Industry has used AC motors as prime movers for many years. The typical AC motor is designed to operate at speeds of 3,000 to 3,600 rpm or more. Many industrial applications and processes utilizing AC motors must reduce the motor speed, typically through mechanical gearing, before the power supplied by the motor can be used. In the oil field, for example, conventional top drive drilling systems may utilize a variable frequency AC motor as the prime mover. The AC motor is responsible for, among other things, supplying the torque needed to rotate the drill string and drill bit during drilling operations. The rotational speed of the bit and, therefore, the top drive is relatively low, typically between 100 and 300 rpm and more typically around 150 rpm. Conventional variable frequency AC motors used in top drive applications have motor speeds usually ten times the bit speed or 1000 to 3000 rpm. Thus, it is conventional for a top drive system to include speed reduction equipment, such as a gearbox, to reduce the rotational speed of the AC motor to a more usable rotational speed. Speed reduction equipment, such as the gearbox mentioned above, adds to the cost of the system, increases the service requirements, increases the number of parts that can fail, and increases the weight of the system.
This application for patent discloses a novel top drive drilling system using a low speed, variable frequency AC motor that does not require additional speed reduction equipment to achieve a rotational speed in the range of 100 to 300 rpm and is, therefore, particularly suited for direct drive applications.